This invention relates to a head positioning control system adapted to be used in a disk drive such as a hard disk drive. More particularly, the present invention relates to a head positioning control system having a compensation feature for suppressing position errors of the head caused by synchronous components that are synchronized with the revolution of a head.
Conventionally, a hard disk drive (HDD) is adapted to make a magnetic head (hereinafter referred to simply as head) write data on and read data from a disk that is a data storage medium. An HDD is typically designed to operate with one or more than one disks at a time and have a pair of heads arranged respectively vis-a-vis the front and rear surfaces of each disk (the respective data carrying surfaces of the disk). In short, an HDD has a pair of heads for each disk. For the purpose of simplification, an HDD having a head for a single disk will be envisaged in the following description.
A disk has a number of concentric tracks (cylinders) on each of its opposite surfaces as data recording area. Each track is divided into a plurality of data sectors (area for recording user data) and provided with servo areas carrying servo data recorded thereon and arranged at predetermined intervals. Each data sector is arranged between two servo areas.
Each set of servo data include a track address (cylinder code) for identifying the track where it is located and servo burst data (position signal pattern). The area from a servo area to the may be referred to as servo sector. Then, each track is divided into a plurality of servo sectors (the number of which corresponds to that of servo areas). Each servo sector comprises one or more than one data sectors led by a servo area.
The HDD is provided with a head positioning control system, which is so configured that, once the track to be accessed (target position) is specified, it controls and drives the head to move to the target position (seek control) in such a way that the head is positioned within the track (track following control). The head positioning control system carries out its track following control operation by means of a feedback control system 1 as conceptually illustrated in FIG. 6 of the accompanying drawing.
The feedback control system 1 basically comprises a position error detecting element (subtracting element) 2, a control element (controller) 4 and a plant (object of control) 3. The subtracting element 2 computationally determines the position error (E) of the actual head position (Y) from the target position (R). The plant 3 is an actuator mounted with a head in a broad sense of the word and a voice coil motor (VCM) for driving the actuator in a narrow sense of the word. The actuator is driven by the VCM to move the head radially on a disk and place the head on the target position.
The controller 4 receives the head position (Y), the target position (R) and the position error (E) as inputs and computationally determines a control value (Ub) to be used for dissolving the position error (E). The control value (Ub) refers to a manipulative control value required for manipulating the drive current of the VCM, which is the plant 3. The subtracting element 2 and the controller 4 specifically refer to a micro controller (CPU) or a main control unit to be used for an HDD. The transfer functions of the controller 4 and the plant 3 are denoted respectively by xe2x80x9cFbxe2x80x9d and xe2x80x9cPsxe2x80x9d.
The controller 4 computationally determines a control value (Ub) so as to carry out an operation of stabilizing compensation for a closed loop system and compensate the deviation of the servo data. The term xe2x80x9cstabilizing compensationxe2x80x9d refers to an operation of stabilizing a loop by compensating the phase delay of a system and hence is also called as xe2x80x9cphase lead compensationxe2x80x9d. To be more accurate, when the phase delay goes below xe2x80x9cxe2x88x92180 degreesxe2x80x9d with a gain crossover frequency, the control loop becomes unstable. Thus, a digital filter is arranged within the control loop to make compensation so as to lead the phase with the gain crossover frequency. On the other hand, the term xe2x80x9cdeviation compensationxe2x80x9d refers an operation of reducing the relative error between the servo data recorded on a track of a disk and the head to improve the accuracy of positioning the head. Since the actuator mounted with the head is connected to an FPC (flexible printed circuit board), it is subjected to the distortions of the FPC and external force. The FPC is provided to transmit signals between the head and the head amplifier and conduct electricity to the VCM. The servo data stored on the disk are subject to positional shifts due to oscillations of the tracks caused by a positional deviation of the disk, oscillations of the spindle motor (disk rotating mechanism) and synchronous and asynchronous oscillations caused by a rotary motion and oscillations of the spindle motor. Thus, an integral type digital filter is arranged within the control loop in order to reduce the relative error between the servo data and the head that is attributable to external force applied to the actuator and positional shifts of the tracks so that an operation of deviation compensation is carried out to improve the accuracy of following a track of the head.
Now, the tracks of the disk where the head is driven for a follow-up motion can give rise to eccentricity due to factors involved in the revolving motion of the disk (eccentricity of the tracks relative to the head). The factors of positional deviation include oscillations of the shaft of the spindle motor for driving the disk to rotate, those of the disk that occur when writing servo data thereon and expansions/ contractions of the disk due to vicissitudes of the ambient temperature.
Known head positioning control systems normally employ a method for improving the effect of suppressing eccentricity of the tracks due to the rotary motion of the disk and hence the head positioning accuracy by raising the gain of the feedback control system. However, in an HDD, the actuator mounted with the head contains elements that can mechanically resonate to make it impossible to raise the gain of the feedback control system.
FIGS. 7A and 7B are graphs showing the spectrum of positioning accuracy (position error) obtained in the operation of positioning the head on a specific track under the control of a conventional head positioning control system. In FIG. 7A, the horizontal axis represents the positions of the servo sectors (the number of which is assumed to be 50 here) of a specific track on the disk when the disk is rotated by a single turn. In FIG. 7A, curved line 140c is obtained by plotting the averages of the observed values. The errors between the curved lines 140a and 140b are attributable to the synchronous eccentric factors that appears in the spindle motor when driving the disk to revolve.
In FIG. 7B, the horizontal axis represents the degrees of eccentric factors (degrees of eccentricity). It will be appreciated that the eccentric factors of the first, second and fourth degrees are not suppressed sufficiently and remain as repetitive periodical factors. In short, known feedback control systems cannot satisfactorily suppress the eccentric factors of the track that become apparent when the disk is driven to revolve.
As described above, known feedback control system cannot satisfactorily suppress the eccentric factors of each track attributable to the revolution of the disk. In an attempt for solving this problem, there has been proposed a head positioning control system combining a feedback control system 1 and a feed forward control system 10, which is a learning system, as shown in FIG. 4.
The feedback control system 10 has a synchronous factor (eccentric factor) detecting section (transfer function (Ft)) 11 adapted to observe the position error (E) at the time of position control operation of the feedback control system 1 and extracts a specific eccentric factor synchronized with the period of revolution of the disk from the position error (E). Feed forward control section (transfer function (Fw) to be referred to as FW controller hereinafter) 12 computationally determines a control value (manipulative control value) Uf for suppressing the synchronous factor (eccentric factor) on the basis of the outcome of the detecting operation (learning achievement) of the synchronous factor detecting section 11 and outputs it to adder section 13 of the feedback control system 1. The adder section 13 adds the control value Ub from the controller 4 and the control value Uf from the FW controller 12 and outputs the obtained result (manipulative control value) to the plant 3.
With such as system having a synchronous factor suppressing function using a learning system, it is possible to suppress the eccentric factors of the track (synchronous factors synchronized with the revolution of the disk in particular, as will be used hereinafter) that the head can hardly follow and control solely by means of a feedback control system 1. Thus, such a system can sufficiently suppress position errors and improve the head positioning accuracy.
FIGS. 5A and 5B are graphs showing the spectrum of positioning accuracy (position error) obtained in the operation of positioning the head on a specific track under the control of such a head positioning control system. As clearly seen from the graphs, a head positioning control system having a feed forward control system 10, which is a learning system, can sufficiently suppress the synchronous factors if compared with a feedback control system 1 having only a feedback control system 1.
However, a head positioning control system having a synchronous factor suppressing function as described above still cannot solve the following problem. That is, the eccentric factors synchronized with the revolution of the disk include synchronous factors that are not correlated among the tracks in addition to the synchronous factors that are correlated among a plurality of tracks of the disk and produced from the environment such as impacts, oscillations and heat emission of the HDD.
More specifically, when recording servo data on the disk (normally by using a dedicated device called servo writer), there appear asynchronous factors of the spindle motor for driving the disk to revolve. Then, it is necessary to detect and suppress synchronous factors that are correlated among a plurality of tracks. However, since a system as described above can only extract the eccentric factors of a single track in a single learning session, a learning session has to be held for each of the tracks and the average of the obtained values has to be determined. Thus, a long period of time will be required to hold learning sessions in order to detect synchronous factors that are correlated among a plurality of tracks of the disk.
It is therefore the object of the present invention to provide a head positioning control system realized by combining a feedback control system and a feed forward control system, which is a learning system, and adapted to reduce the time required for holding learning sessions necessary for detecting synchronous factors that are correlated among a plurality of tracks of the disk (eccentric factors of the tracks synchronized with the revolution of the disk) and efficiently extract and suppress the synchronous factors.
According to the invention, the above object is achieved by providing a head positioning control system realized by combining a feedback control system and a feed forward control system, which is a learning system, and having a functional feature of detecting and suppressing a synchronous factor correlated among a plurality of tracks of a disk by a single learning session of the learning system.
More specifically, according to the invention, there is provided a system for positioning a head in a target position on a disk in a disk drive, the system comprising:
means for computationally determining the position error of the position of a head of a disk drive from a target position by means of servo data recorded on a disk;
first control means for computationally determining a first control value for positioning the head at the target position on the basis of the position error;
means for generating a disturbance and adding the disturbance with a predetermined frequency different from the revolutions per unit time of the disk to the position error;
second control means for detecting a positional eccentric factor synchronized with the revolution of the disk from the position error including the disturbance and determining a second control value for removing the positional eccentric factor from the position error; and
means for controlling and positioning the head at the target position, using a control value obtained by adding the first control value and the second control value.
With such a system, a synchronous factor correlated among a plurality of tracks of the disk can be extracted by a single learning session conducted while moving the head among a range containing a plurality of tracks. Thus, the time required for holding a learning session of detecting a synchronous factor correlated among a plurality of tracks can be reduced. With such a system, an operation of controlling and positioning the head can be realized reliably and efficiently by adding the second control value determined to remove the synchronous factor correlated among a plurality of tracks from the position error to the first control value computationally determined on the basis of the position error.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.